Recent technological advances have enabled radio designers to significantly reduce the size of an RF radio. Such reduction is primarily attributed to very large scale circuit integration. Other radio circuits, which cannot be condensed into an integrated circuit, must be size-reduced using other techniques.
One such circuit is a transmission line power splitting circuit. This circuit is typically used at the output of the radio's voltage controlled oscillator (VCO) to split the power of the VCO's output signal to the receiver local oscillator input, the transmitter local oscillator path and the synthesizer feedback path (for controlling the frequency of the VCO). Such power splitting requires transmission line circuitry which cannot effectively be compacted into an integrated circuit because of size constraints. Hence, there have been significant efforts to design transmission line power splitting circuits which reduce the size of the circuit to accomodate the desired size reduction of the radio.
Of these efforts, one of the more successful combines a capacitive tap with a Wilkinson power splitter at the power signal to provide a 3-way power split. The Wilkinson power splitter typically includes two quarter wave-length transmission lines emanating from a connection at the power signal at one end and separated by an impedance isolation circuit at the other end to provide two output ports. The two output ports respectively provide two paths of split power, while the third path is provided by the capacitive tap.
This configuration is disadvantageous, however, because it fails to provide isolation between the output port at the capacitive tap and the output ports at the Wilkinson splitter. Additionally, the capacitive tap loads the input power signal causing a potential mismatch at the Wilkinson splitter output ports. Further, the quarter wave-length transmission lines of the Wilkinson splitter occupy excessive circuit board real estate. For these reasons, this configuration is often unacceptable.
There are two other common techniques for providing such a 3-way power split. One technique combines two 2-way Wilkinson power splitters, where one splitter is used to split the power provided at one of the output ports from the other splitter. The second technique involves a straight 3-way Wilkinson power splitter circuit, similar to the Wilkinson 2-way splitter but rather split 3 ways directly from the input power signal using three isolation circuits, each of which is connected between each pair of transmission lines. Both of these techniques, unfortunately, have significant disadvantages.
One disadvantage is that both require excessive circuitry to implement the isolation circuit(s). Each isolation circuit, common to each of these techniques, requires at least two discrete parts. The elimination of such discrete parts is a primary object in reducing the size of the radio.
A second disadvantage is that the layout of the quarter wave-length transmission lines and the discrete circuit elements requires excessive circuit board real estate. Although it is well known that the length of the quarter wave transmission lines can be reduced by adding additional capacitors, so doing adds to the number of discrete parts; therefore, defeating the goal of reducing the size of the circuit.
A third disadvantage is that these known techniques for power splitting do not allow flexibility in the power distribution at the output ports of the Wilkinson power splitting circuit.
Accordingly, a technique for splitting the power three ways at the output of a VCO (or similar circuit) is needed which overcomes the aforementioned deficiencies.